Along with recent high-frequency and high-capacity communication, a multi-level phase modulation method such as differential quadrature phase shift keying (DQPSK) is employed in an optical device such as an optical modulator. In addition, integration is in progress in a dual polarization-quadrature phase shift keying (DP-QPSK) modulator in which two DQPSK modulators are polarization-multiplexed, and thus the number of signal electrodes, which are provided in a modulator, increases.